The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system having a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximating the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, an integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the integrating circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The control system operates to determine whether the feedback signal from the oxygen sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal for actuating the on-off type electromagnetic valve so as to control the air-fuel ratio of the mixture.
The response of such a feedback control system is inherently slow because the time of detection by the oxygen sensor is delayed. More particularly, the mixture that is corrected by the on-off type electromagnetic valve is induced in the cylinders of the engine passing through the induction passage and burned therein, and thereafter discharged into the exhaust passage. Therefore, by the time the oxygen sensor detects the oxygen content of the exhaust gases based on the corrected mixture, the corrective action with the on-off electromagnetic valve has overshot the desired point. As a result, a rich or lean mixture caused by the overshooting is induced in the engine and the deviation of the air-fuel ratio is detected by the oxygen sensor. Thus, a corrective action in the opposite direction will be initiated. After such an oscillation of the control operation, the variation of the air-fuel ratio of the mixture will converge toward the stoichiometric ratio. Therefore, the deviation of the air-fuel ratio of the mixture is corrected to the stoichiometric ratio with some delay. Consequently, the desired reduction of the harmful constituents may not be achieved.
Japanese Patent Application No. 54-98853 (U.S. Patent application No. 174,385 filed on Aug. 1, 1980, not prior art, but the disclosure thereof being incorporated by reference herein) discloses a system intended for improvement of such a control delay, in which the oscillation center of a dither wave signal detected by the oxygen sensor is shifted according to the deviation of the output signal of the oxygen sensor for correcting the air-fuel ratio. However, when the engine is rapidly accelerated or decelerated, the correcting operation is delayed even in such a system.